Precise cooperative sulfur placement leads to semi-crystallinity and selective depolymerisability in CS2/oxetane copolymers

CS2 promises easy access to degradable sulfur-rich polymers and insights into how main-group derivatisation affects polymer formation and properties, though its ring-opening copolymerisation is plagued by low linkage selectivity and small-molecule by-products. We demonstrate that a cooperative Cr(III)/K catalyst selectively delivers poly(dithiocarbonates) from CS2 and oxetanes while state-of-the-art strategies produce linkage scrambled polymers and heterocyclic by-products. The formal introduction of sulfur centres into the parent polycarbonates results in a net shift of the polymerisation equilibrium towards, and therefore facilitating, depolymerisation. During copolymerisation however, the catalyst enables near quantitative generation of the metastable polymers in high sequence selectivity by limiting the lifetime of alkoxide intermediates. Furthermore, linkage selectivity is key to obtain semi-crystalline materials that can be moulded into self-standing objects as well as to enable chemoselective depolymerisation into cyclic dithiocarbonates which can themselves serve as monomers in ring-opening polymerisation. Our report demonstrates the potential of cooperative catalysis to produce previously inaccessible main-group rich materials with beneficial chemical and physical properties.


Supplementary Notes 1: Supplementary methods
Solvents and reagents were obtained from commercial sources and used as received unless stated otherwise. Oxetane in particular was obtained from Sigma Aldrich as well as Fisher Scientific. NMR spectra were recorded by using a Jeol JNM-ECA 400II, Bruker Advance 600 and 700 MHz spectrometer. 1 H and 13 C{ 1 H} chemical shifts are referenced to the residual proton resonance of the deuterated solvents.
Oxetanes, CS2, PO and CHO were dried over calcium hydride at room temperature for 3 days followed by vacuum transfer (for CHO fractionally distilled under static vacuum) and three freeze pump thaw degassing cycles and stored inside an argon filled glovebox prior to use. Oxetane employed in table 1 run #8, #9 and #10 was additionally dried over elemental sodium. PPNOAc, KOAc@18-crown-6 and A were prepared following literature methods. [1][2][3] Cyclohexanediamine was dried over 4Å molecular sieves and degassed prior to use. A and L' were synthesized according to the literature procedure. [2,4] DCM was dried by using an MBraun Solvent Purification System MB-SPS 800 filled with Al2O3 followed by drying over 4Å molecular sieves. THF was dried over K/benzophenone under argon followed by drying over 4Å molecular sieves. Benzylalcohol and mesitylene were dried over 4Å molecular sieves. All other reagents were used as received if not stated otherwise.
High-resolution mass spectra were obtained using a Waters UPLC-Synapt G2-S HDMS. Infrared spectra were measured using a Thermo-Nicolet Nexus 670 FTIR spectrometer with DuraSampl IR accessory in total reflection at room temperature. TGA data was measured using a Netzsch TG 209 (heating rate 10 K/min). DSC was measured on a Netzsch 204 F1 "Phoenix". PXRD was measure in a Malvern Panalytical Empyrean diffractometer equipped with a PIXcel 1D detector using Cu-Kα radiation (λ = 1.54 Å) at room temperature in reflection mode. Films were prepared by hot-pressing 500 mg of polymer between two aluminium plates covered with Teflon sheets heated with two LLG hotplates (held in place by a 5 kg weight put on top) at 110ºC for 5 minutes followed by hardening at 70ºC for 2h. Storage modulus (E′, MPa), loss modulus (E″, MPa) and tan δ (E″/E′) measurements were performed on a DMA 242 C (NETZSCH) in tension mode on rectangular specimens (l = 7.650 mm, w = 3.810 mm, t ≈ 0.719 mm). The stamps were tightened to 10 Nm with a torque wrench. Temperature-ramp experiments were performed at 1 Hz between -100 and 90 °C at a heating rate of 1 K/min. Temperatures below ambient conditions were accessed via liquid nitrogen CC 200 supply system (NETZSCH). Post-run analysis was performed on a NETZSCH Proteus Thermal Analysis software. Uniaxial tensile testing was performed on a Wick/Roell Z010 instrument (ZwickRoell GmbH & Co., KG, Germany, 500 N load cell class 0.5, extensimeter multixtens class 0.5).
The molecular mass and polydispersity of the polymers were determined by a Waters 1515 gel permeation chromatography (GPC) instrument equipped with two linear PLgel columns (Mixed-C) following a guard column and a differential refractive index detector using tetrahydrofuran as the eluent at a flow rate of 1.0 mL/min at 30 °C and a series of narrow polystyrene standards for the calibration of the columns. Each polymer sample was dissolved in HPLC-grade THF (6 mg/mL) and filtered through a 0.20 μm porous filter frit prior to analysis.
Molecular periodic calculations for the systems in figure 5 were performed with the CRYSTAL17 program, [8] using the B3LYP DFT functional in combination with Grimme type dispersion correctionand employing the Gaussian-type atomic basis set cc-pVDZ. [9][10][11][12] The first Brillouin zone was sampled using an 6×6×6 Monkhorst-Pack grid. To facilitate convergence, the Coulomb and exchange integral thresholds were sufficiently tightened with the TOLINTEG keyword to values of 8, 8, 8, 8 and 16. For the calculations of dimers of oligomer chains, the calculations were performed at the same level of theory but with the Gaussian programAIM analysis according to Bader was performed with the Multiwfn program. [13,14] Supplementary  (Table 1, run #1), DCM was used to solubilize the reaction mixture out of the vial resulting in trace ammounts. Assignment according to 2D NMR spectra of scrambled polymers of Section S5 and in reference to [5] . Assignment by HMBC (vide infra) and in reference to [5] .

Supplementary
Supplementary Figure 25: Zoom into the heterocarbonate region of the 13 C NMR spectrum (126 MHz, CDCl3, 25°C) spectrum of the product mixture corresponding to table 2, run #4. Assignment by HMBC (vide infra) and in reference to [5] . Inside an argon filled glove-box, LCrK (1 eq.), poly(trimethylene dithiocarbonate) (100 eq. repeat unit prepared as per table 1, run #1) and oxetane (1000 eq.) were added to a flame dried vial equipped with a flame dried stirrer bar and sealed with a melamine cap containing a Teflon inlay. The vial was brought outside the glovebox and placed in a pre-heated aluminium block at 80ºC for 16h. Afterwards, oxetane was removed by air-drying, the reaction mixture was taken up in CDCl3 and analysed by NMR. Furthermore, the products were analysed by GPC.

Supplementary Notes 8: De-and repolymerisation experiments
In analogy to reports by Endo and co-workers the polymer (1 eq.) was dissolved at 0.45 M concentration (treating the repeat unit as one entity) in THF containing 0.01 eq. KO t Bu and let react at room temperature. [7] The reaction was monitored by 1 H NMR analysis of aliquots and stops after completion. Afterwards all solvent was removed in vacuum to obtained the cyclic dithiocarbonates as yellow (semi)solids.  Figure 100: Repolymerisation by ROP.

Supplementary
Repolymerisation procedure: C T (660.0 mg, 25 eq.) was dissolved at 4 M concentration in dry DCM and benzylalcohol (4.2 µL, 1 eq.) and 1,5,7-triazabicyclo[4.4.0]dec-5-en (5.7 mg, 1 eq.) were added. The resulting mixture was stirred for 16h at room temperature and then added to 45 mL of MeOH acidified with a few drops of aqueous HCl resulting in the precipitation of the product. The product was redissolved in 1mL DCM and precipitated into pentane and dried in a vacuum oven set to 40ºC overnight to yield the polymer as a sticky yellow solid (300 mg, 45% yield).
Supplementary Figure 101: 1 H NMR spectrum (400 MHz, CDCl3, 25°C) of the polymer obtained from repolymerisation experiment. Assignment according to links observed in CS2/OX Me copolymer.

Supplementary Notes 10: Density functional theory
The individual association energy per unit in reference to Figure 6